The present invention relates to porous inorganic membranes having extremely small pore sizes and a method for producing the fine-pored porous inorganic membranes. These porous inorganic membranes are especially useful in processes for the separation of different size molecules in gases or liquids at high temperatures and in harsh chemical environments, such as are encountered in coal gasification processes and in the petrochemical industry. The United States Government has certain rights to this invention pursuant to Contract No. DE-AC05-84OR21400 with Lockheed Martin Energy Systems, Inc. awarded by the U.S. Department of Energy.
Development work has been carried out at a number of international locations with respect to the problem of producing porous inorganic membranes, such as ceramic membranes, having extremely small pore sizes, i.e. pores having pore diameters of a few Angstroms. Such inorganic membranes are needed for use in the separation of gases at high temperatures and in harsh chemical environments, such as are encountered in coal gasification processes and in the petrochemical industry. In particular, the membrane pores must be sufficiently small to separate gas molecules on the basis of molecular size, in a process usually referred to as molecular sieving, in order to achieve high separation factors.
Various prior art techniques have been investigated for preparing inorganic membranes, which may be either porous or nonporous in physical makeup. The latter category is typified by palladium or silver foil metals. See Ceramic Membranes for Gas Separation, xe2x80x9cSynthesis and Transport Propertiesxe2x80x9d Robert Jan Reinier Uhlhorn, pp 3-5, November 1963. Our invention relates to the porous category of inorganic membranes and in particular to metal oxides, metal carbides, metal nitrides, and cermets.
Membranes may be generally classified by the size of the molecules or particles being separated and generally fall into four broad categories: reverse osmosis (average mean pore diametersxe2x80x941 xc3x85-10 xc3x85), ultrafiltration (average mean pore diametersxe2x80x9410 xc3x85-1000 xc3x85), microfiltration (1000 xc3x85-10,000 xc3x85), and particle filtration ( greater than 10,000 xc3x85). More recently, research has been performed on nanofilters which include the upper molecular weight range of the reverse osmosis domain and the lower molecular weight range of the ultrafiltration domain.
The porous inorganic membranes typically are composed of a porous support or carrier with a thin separation layer. Further, the porous inorganic membranes are housed in modules having various configurations, such as hollow-fibers, spiral wound and plate-and-frame or flat-sheet configurations. See Emergina Separation and Separative Reaction Technologies for Process Waste Reduction, Peter P. Radechi et al, pp. 17-18, Center for Waste Reduction Technoloies American Institute of Chemical Engineers, New York, N.Y., 1999.
The prior art for making porous inorganic membranes, which has a market value of in excess of $500 million, is obviously quite extensive. However, new development in the inorganic membrane field is expected to increase the value by a factor by at least 10 fold. Of the prior art methods for preparing inorganic membranes, the one that is the most extensively used is a process commonly called the xe2x80x9csol-gelxe2x80x9d process, which has been used to prepare inorganic membranes.
The sol-gel process is basically the use of a colloidal suspension of various metal oxides or other ceramic materials to make ceramic articles, which are either porous or non-porous. Typical materials are alumina, silica, titania, zirconia, or mixtures thereof. The colloidal suspension is formed by various precipitation methods. In general, the colloidal particles are very small, e.g., 1000 xc3x85 to reported as small as 30 xc3x85. When a sufficient amount of the liquid (mostly water) is removed, the colloidal suspension (or sol) becomes a gel. To make an article, the sol-gel is formed, further dried, calcined and sintered. Depending on the degree of sintering, the article can be porous to various degrees or can approach full density.
When used to make membranes, in most cases, a porous article is desired. The size of the pores in the membrane is determined by the size and uniformity of the particles. The pores are the interstices between the particles. The effective diameter of the pores is approximately one half the diameter of the particles. If one could make a suspension with 30 xc3x85 particles (and that is really difficult), about the smallest expected pore diameters would be 15 xc3x85 or larger.
The void fraction (or fraction of the membrane that is pores) of sol-gel membranes is about 50% more or less (but not much). For a membrane with such small pores to have any practical use, it must be very thin, i.e., a few microns or preferably less. Membranes are made by applying a thin layer of the sol to the surface of a porous support material. A simple way would be to pour the sol onto the surface and then allow most of it to drain off (or by other means) to remove most of it. Initially, the sol is pulled into the surface pores of the porous support by capillary action. The sol stays near the surface and the water is pulled into the interior by capillary action. This removes a large fraction of the water from the sol and causes it to gel.
An important factor in achieving thin membranes is to have the pore diameter of the support material to be less than 100 times the expected pore diameter of the membrane. This may require a porous support with one or more intermediate layers.
It is difficult to dry and calcine the membrane layer without having a significant number of cracks (defects) in the membrane layer. The smoother the surface of the support material the fewer the cracks. This problem is frequently solved by applying several layers of the sol so that cracks that do result will be covered by one or more of the layers. See Emerging Separation and Separative Reaction Technologies for Process Waste Reduction, above, for additional details of the sol-gel method for producing porous inorganic membranes.
While the class of inorganic membranes commonly called xe2x80x9czelolitesxe2x80x9d have been prepared with pore sizes in the few Angstrom range, these membranes have a fundamental different physical structure than the typical porous inorganic membrane, such as a metal oxide. The crystallographic structure of a zeolite defines the pore diameters in contrast to a ceramic membrane wherein the pores are the interstices between the particles. Thus, while the zeolites represent an interesting approach to ceramic membranes, the basic problem to use of the zeolites as membranes in industrial applications is that the zeolite particles have to be grown into a membrane; it is difficult to grow them thin enough without defects, which without a major breakthrough limits their commercial or industrial utility.
Currently, no porous inorganic membranes having sufficiently small pores are commercially available for molecular sieving types of gas separation applications.
There is a need to provide porous inorganic membranes that have mean diameter pore sizes on the order of several Angstroms, i.e., below about 20 xc3x85 for use in separating molecules based on their size. Also, there is a need to provide an efficient method for preparing extremely small-pored inorganic membranes and in particular to provide a method that lends itself to commercial scale operations.
One objective of this invention is to provide a porous inorganic membrane having a mean pore diameter about 20 xc3x85 or less.
Another objective is to provide a process for producing extremely fine-pored inorganic membranes that are suitable for a wide range of industrial uses, including recycle of hydrogen in petroleum refinery, higher yields in olefin production and improved efficiency in a large number of chemical separation processes.
Still a further object is to provide a method for controlling the reduction of the pore diameter of porous inorganic membranes to tailor the resultant membrane for specific industrial uses for separating specific different size molecules.
In accordance with the above and other objects of the invention, it has been found that fine-pored inorganic membrane comprising a matrix of material particles having at least one monolayer of an inorganic compound uniformly deposited on the surface of the particles which make up the pore walls of the matrix can be prepared in which the mean pore diameter of the pores are less than about 20 xc3x85. In one embodiment the porous inorganic membrane, for example, comprises an inorganic matrix of metal oxides, metal carbides, and metal nitrides with at least a monolayer of an inorganic compound, selected from the group consisting of metal oxides, metal carbides, and metal nitrides, uniformly deposited on the pore walls of the inorganic membrane.
We have found, quite unexpectedly, that a process for controlling the ultimate pore size of an fine-pored inorganic membrane could readily be achieved by depositing one monolayer at a time of an inorganic compound, such as a metal oxide, metal carbide, or metal nitride on the pore walls of the inorganic membrane. Accordingly, with each layer of the inorganic compound a effective reduction in mean pore diameter of the inorganic membrane product of a thickness of approximately one molecule of the inorganic compound, e.g., for gamma-phase Al2O3 a thickness of about 2.5 xc3x85. In addition, as the monolayers are applied, the individual particles grow together forming a continuous matrix
The process can, advantageously, be repeated one layer at a time to reduce the pore size of the matrix of the inorganic compound to achieve mean pore diameter of the pores to below about 20 xc3x85 and even below 5 xc3x85. The resulting fine-pored inorganic membranes are especially useful for gas separations, including a range of applications involving high temperature and harsh environments, such as, for example, the separation of hydrogen from gasified coal at process temperature.
Also, these fine-pored inorganic membranes are useful in large-scale industrial applications in, for example, the petroleum industry and include the separation of hydrogen from high-temperature catalytic dehydrogenation processes used for a broad range of petrochemicals, such as olefin production, as well as the removal of hydrogen from the refinery purge gases. These inorganic membranes with their extremely small pore size and uniformly deposited inorganic compound on the walls of the pores of the matrix, which have heretofore not been attained by the prior art, are uniquely useful as membranes, including molecular sieves.
In accordance with the invention, a method is provided for producing porous inorganic membranes having the extremely small pore sizes discussed above, i.e., pore sizes capable of providing separation of gas molecules by molecular sieving.
According to the invention, a method is provided for reducing the pore size of a porous inorganic membrane having a surface including therein pores with pore walls, wherein the method comprises depositing at least one layer of inorganic compound on the pore walls of the pores of the inorganic membrane, with each layer of inorganic compound deposited on the inorganic membrane having a thickness of approximately one molecule. Preferably, the depositing of the at least one layer comprises depositing a sufficient number of layers to reduce the mean pore diameter of the pores to 20 xc3x85 or less.
In one embodiment of the present invention the pore size of the matrix of an inorganic compound is reduced by vapor treating the inorganic compound with a reactive vapor of a inorganic precursor compound which (1) includes a reactive group that reacts with surface hydroxyls or other surface molecules on the inorganic membrane and which (2) also reacts with water or other chemical vapor that can combine with the precursor to produce a surface that the precursor will react with. This reactive vapor produces a reaction with the surface hydroxyls on the inorganic membrane surface to bond precursor molecules to the inorganic membrane. Preferably, the inorganic membrane surface is thereafter treated with water vapor, oxygen, or vapors containing one or more oxygen ligands such as an alcohol to convert the inorganic precursor compound into the corresponding inorganic compound.
The membrane is preferably treated with an inorganic precursor compound selected from the group consisting of chloro-silanes, organo-silicon compounds, chloro-titaniums, organo-titanium compounds, organo-aluminum compounds, and chloro-zirconia, and organo-zirconia compounds. Further, the inorganic membrane is preferably made of a inorganic compound selected from the group of alumina, titantia, zirconia, silica and alumina/silica mixtures.
Advantageously, the method further comprises drying the inorganic membrane prior to treating the membrane with the reactive vapor of the inorganic precursor compound. This drying preferably comprises heating the membrane and holding the membrane at temperature of 100xc2x0 C. to 200xc2x0 C. for one to two hours in an evacuated vessel. In this implementation, the treating of the inorganic membrane with the reactive vapor of the inorganic precursor compound preferably comprises introducing the reactive vapor into the evacuated vessel, evacuating the vessel to remove unreacted inorganic precursor compound products and then introducing the water vapor, oxygen, or vapors containing one or more oxygen ligands such as an alcohol into the vessel. Advantageously, the method further comprises evacuating and refilling the vessel alternately with the reactive vapor and water vapor a plurality of times.
In one preferred implementation, the inorganic membrane is comprised of alumina and the vapor treating with a reactive vapor comprises treating with a trimethyl aluminum vapor. In an alternative implementation wherein the inorganic membrane is comprised of alumina, the treating with a reactive vapor comprises treating the inorganic membrane with an anhydrous aluminum chloride vapor while in another implementation, the treating with a reactive vapor comprises treating the inorganic membrane with a titanium tetrachloride vapor.
In an advantageous embodiment, the at least one layer is deposited only on one side of the inorganic membrane. Preferably, prior to depositing the at least one layer, the inorganic membrane is seated in a holder, which enables deposition on only the one side.
Other features and advantages of the invention will be set forth in, or apparent from, the following detailed description of preferred embodiments of the invention.